This invention relates to improvements in NOx reduction catalyst and NOx reduction system for reducing NOx in exhaust gas discharged from a combustion device such as an internal combustion engine, a combustor, a boiler or the like, and more particularly the NOx reduction catalyst and NOx reduction system for reducing NOx as well as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) contained in exhaust gas discharged from an automotive internal combustion engine in such a manner that NOx can be particularly effectively reduced in a lean region of exhaust gas and that SOx can be adsorbed to the NOx reduction catalyst in a range including a stoichiometric region and a rich region.
In recent years, low fuel consumption automotive vehicles have been required from the viewpoints of petroleum resource exhaustion problem and warm-up problem for the earth. In this regard, attentions have been paid on development of lean-burn automotive engines. In the lean-burn automotive engines, exhaust gas (atmosphere) becomes in a lean region in which air-fuel ratio of exhaust gas is lean as compared with a stoichiometric level, during lean-burn operation. In case that a usual thee-way catalyst is used in such a lean region of exhaust gas atmosphere, reduction of NOx can become insufficient under the effect of excessive oxygen. Consequently, it has been eagerly desired to develop exhaust gas purifying catalysts which can effectively reduce NOx even in the presence of excessive oxygen.
In view of the above, a variety of exhaust gas purifying catalysts for reducing NOx in the lean region of exhaust gas have been proposed. An example of these exhaust gas purifying catalysts is disclosed in Japanese Patent Provisional Publication No. 5-168860, in which platinum (Pt) and lanthanum (La) are carried on a porous substrate so that NOx is trapped in the lean region of exhaust gas and released in a range including the stoichiometric region and the rich region.